Flow cytometry and sorting is a methodology which provides a unique approach to the study of atherosclerosis. The focal nature of atherosclerosis and the low numbers of foam cells (i.e., cholesteryl ester-rich cells) occurring in spontaneous or early experimentally induced lesions precludes determinations of foam cell numbers by other means. This research has demonstrated that foam cell number can be quantified using flow cytometry. Six-month-old male swine were maintained for 6 months on control, lard, or cholesterol-lard diets. Some animals were fed the cholesterol-lard diet for 3 months and then fed the control diet for an additional 3 months. Other animals studied were comprised of male and female swine up to l2 years of age that had been maintained on a cholesterol-free diet their entire lifetimes. Intimal-medial aortic tissue preparations were enzymatically digested using collagenase and elastase. The resulting single-cell suspensions were fixed in formalin. A technique for fluorescent staining of the cholesteryl ester-containing foam cells in these cell preparations was developed. The fluorescence of each of l00,000 stained cells was measured using a flow cytometer. Flow cytometric analysis and sorting of these filipin-stained cell preparations showed that foam cells could be quantified and purified. Animals never fed cholesterol had foam cells. Foam cells increased in number in cholesterol-lard-fed animals but did not increase in lard-fed or control-fed animals. Foam cells did not continue to increase after cholesterol-lard-fed animals were switched to control diets. The finding that animals never fed cholesterol, nevertheless develop foam cells is highly significant. Also significant is the finding that feeding of cholesterol but not feeding of lard (i.e., saturated fat) accelerates the development of foam cells. This accelerated development can be reversed when cholesterol is removed from the diet.